Properly handle computed fields

[vkleen]

Computed fields in terraform come in two types. One is never settable by the user configuration and should be handled similarly to the id field hack we have right now. That is, an extra field with forced value should be set.

Others are settable by the user and should be handled similarly, but with a default value.

[vkleen]

Determining the interpolation type is possible by carefully parsing the provider schema.

A tricky issue is dealing with operations on interpolated fields. E.g. if a field foo has type a list of strings, we would like to access it as a list in Nickel. But setting foo to a default value "${foo}", so that recursive access in Nickel is possible, doesn't allow one to observe that foo should actually be a list.

[vkleen]

Sometimes terraform configuration blocks need to reference themselves. HCL uses a self keyword for that. The documentation indicates that in the example below using ${aws_instance.web.public_ip} instead of ${self.public_ip} would not be accepted.

resource "aws_instance" "web" {
  # ...

  # Establishes connection to be used by all
  # generic remote provisioners (i.e. file/remote-exec)
  connection {
    type     = "ssh"
    user     = "root"
    password = var.root_password
    host     = self.public_ip
  }

  provisioner "remote-exec" {
    inline = [
      "puppet apply",
      "consul join ${aws_instance.web.private_ip}",
    ]
  }
}